The next-generation wireless communication system LTE-A (Long Term Evolution-Advanced) of 3GPP requires providing a peak rate of 1 Gps and a peak spectrum efficiency of 30 bps/Hz in the downlink. This brings challenge to the transmission scheme in the physical layer of the system. A multi-antenna MIMO (Multiple Input Multiple Output) system is able to support parallel data flow sending thereby greatly increasing the system throughput. Typically, the independent forward error correction encoding is firstly performed on the parallel data flow in the multi-antenna transmission, and then the encoded code words are mapped into the corresponding data transmission layer. In one transmission, the number of all the layers supported by the system is also referred to as a Rank of this transmission. The process of transforming data in each layer into data on each physical antenna is referred to as a pre-encoding process for a signal. LTE-A Rel-10 supports a pre-encoding technology with maximum Rank of 8.
The sending terminal should transmit pilot sequences used for channel estimation, namely demodulation reference signals (DMRSs), for the receiving terminal to perform MIMO decoding and related demodulation. The design of DMRSs should satisfy that DMRSs corresponding to each data transmission layer are mutually orthogonal, i.e. ensure that there is no interference between equivalent channels of pre-encoded channels of respective sending antennas. In a Rel-10 system, DMRSs corresponding to each data transmission layer are distinguished in the manner of frequency division multiplexing (FDM) and/or code division multiplexing (CDM). The code division multiplexing is implemented by spreading sequences whose correlation is ideal with orthogonal cover code sequences. The orthogonal cover code sequences usually employ Walsh Code sequences or Discrete Flourier Transform sequences.
If the orthogonal cover code sequences are mapped in the time domain, i.e. spread in the time domain, it is usually assumed that the channels in the physical resources corresponding to the cover code sequences are identical. Assuming that a spreading factor of a spreading sequence is M, the channel response of the M OFDM symbols are considered to be identical. This assumption is true in the low speed environment. However, with the increasing moving speed of a mobile station, variations of the channel response of the M OFDM symbols increase and the orthogonality of the spreading codes are destroyed, leading to mutual interference between respective data transmission layers and thus reducing the accuracy of the channel estimation.
Moreover, in the Rel-10 system, DMRSs are subjected to the same pre-encoding process as that for data and are mapped onto each sending antenna. The pre-encoding process performs linear superposition on the DMRSs corresponding to each of the code division multiplexed data transmission layers. If the DMRSs corresponding to the M data transmission layers are superposed in the same direction, a signal with amplitude of M is gotten; and if the DMRSs corresponding to the M data transmission layers are superposed in the opposite direction, they are mutually canceled out and a signal with amplitude of 0 is gotten. If such power imbalance of each of the sending antennas occurs in the entire frequency bandwidth, the efficiency of the transmission power may be reduced apparently.
The reference documents of the present invention are listed in the following, which are incorporated herein by reference as if they are described in detail in the present description.
1. [Patent Document 1]: Ishii Hiroyuki, Higuchi Kenichi, Base station apparatus, user apparatus and method used in mobile communication system (US 20100034077 A1);
2. [Patent Document 2]: Hooli Kari, Pajukoski Ka, et al., Method, apparatuses, system and related computer product for resource allocation (WO 2009056464 A1);
3. [Patent Document 3]: Kim Hak Seong, Yun Young Woo, et al., Method of transmitting scheduling reference signal (US 20100008333 A1);
4. [Patent Document 4]: Che Xiangguang, Guo Chunyan, et al., Variable transmission structure for reference signals in uplink messages (WO 2009022293 A2);
5. [Patent Document 5]: Cho Joon-young, Zhang Jianzhong, et al., Apparatus and method for allocating code resource to uplink ACK/NACK channels in a cellular wireless communication system (US 2009046646 A1);
6. [Patent Document 6]: Yang Yunsong, Kwon Younghoon, System and method for adaptively controlling feedback information (US 20090209264 A1); and
7. [Patent Document 7]: Pajukoski Kari P, Tiirola Esa, Providing improved scheduling request signaling with ACK/NACK or CQI (US 20090100917).